1. Field
Aspects of the present disclosure relate generally to a comparator circuit. More particularly, this disclosure includes a low power, single-ended comparator circuit capable of receiving high-voltage input signals, where the circuit is implemented using low-voltage semiconductor devices.
2. Background
With the evolution of MOS semiconductor technology, transistor feature size is becoming smaller in order to increase circuit operating frequency and improve power efficiency. On the other hand, decreasing transistor dimensions requires decreasing power supply voltage and CMOS gate-oxide thickness so that constant electrical field scaling can be realized. Thus, as MOS semiconductor technology has evolved, the maximum tolerable voltage across transistor gate oxide in newer semiconductor devices has become smaller. However, some communication protocols still require electrical signal levels that are higher than those that these transistors are capable of supporting. For example, in standards such as the Universal Serial Bus (USB) standard as promulgated by the USB Working Group, single-ended data signals have a voltage swing of 0-3.3V. However, 3.3V-tolerable transistors are not available in many present advanced deep sub-micron CMOS processes. Although a conventional over-voltage protection transistor may be used to create a circuit that may handle the required voltage swing by reducing the level of the input signal, the voltage level at the upper end of the signal as reduced by the conventional over-voltage protection transistor may not be large enough to reach a required switching threshold of a subsequent circuit. Thus, it is becoming more difficult to integrate circuits that support these higher voltage communication protocols into newer semiconductor devices. Consequently, multiple semiconductor devices must be fabricated, which drives up costs and complexity.
A circuit capable of handling high-voltage signals using only low-voltage transistors is desirable to allow integration of circuits that are specification compliant with communication protocols requiring high-voltage signals into the same die as other circuits not having these requirements.